1. Field of the Invention
The present invention generally relates to underwater pelletizing equipment and a method of processing and pelletizing polymeric resins and similar materials. More specifically, the present invention relates to underwater pelletizing equipment and a method of processing and pelletizing polymeric resins and other extrudable materials in which the melt cooler and associated valving can be utilized to maximum efficiency for the different polymeric resins being processed and pelletized.
2. Description of the Prior Art
One known production process has been commonly used for many years for a broad array of hot melt and pressure sensitive adhesive products made from such polymer resins as ethylene vinyl acetates (“EVA”), polyethylenes (“PE”), polypropylenes (“PP”), thermoplastic elastomers (“TPE”), thermoplastic urethanes (“TPU”), polyesters and polyamides as their base ingredients, and as combined with many other materials, such as waxes, tackifiers, pigments, mineral fillers, antioxidants, etc. This known process has also been successfully applied to other non-adhesive products such as gum bases, varieties of chewing gum, and asphalts.
The aforementioned process can be applied to nearly any polymer application in which the product is made, blended, mixed, or compounded, usually at a relatively high temperature, and then which must be cooled considerably in order to have a more suitable condition just prior to passing through a die plate and then being cut into pellets. Pellets are the most common and desired form for the packaging, transporting, and subsequent handling, blending, melting, molding, and overall use of such aforementioned polymeric materials.
The aforementioned known production process generally consists of the following processing components, as shown in FIG. 1 of the accompanying drawings: reactor, mixing vessel or extruder 1; melt pump 2; filter 3; melt cooler with dedicated heat transfer fluid system 4; polymer diverter valve 5; die and pelletizer 6 (with optional bypass piping); tempered water system 7 (with optional water filtration equipment); water separator/dryer 8 (with optional pellet screening equipment); and conveying and/or packaging equipment 9.
The melt cooler 4 is basically a heat exchanger, of which there are many types, such as, for example, plate and frame, shell and tube, scraped wall, etc. The melt cooler 4 lowers the melt temperature of the polymer or extrusion product passing through the cooler. However, some types of melt coolers are more efficient than other types, with the primary focus being to most efficiently remove heat energy. But many other functional considerations are important to this component of the overall apparatus and method. For example, some of the considerations associated with the melt cooler include: minimizing pressure drop of the melt; process considerations associated with the elevated process temperatures and pressures; materials of construction considerations associated with the elevated process temperatures and pressures; ease of cleaning; minimizing floor space occupied by the cooler and piping; and providing the flexibility to either cool or heat a product, depending upon the specific processing service.
The aforementioned prior art process that is most commonly utilized has a melt cooler of a single pass shell and tube design combined with static mixer elements, as shown in FIG. 2. The melt cooler 10 shown in FIG. 2 achieves good results when working with either a specific product or with a wide variety of products. However, many polymer producers have a broad array of polymer products, including some products that need not be cooled prior to pelletizing. Thus, the step of pumping those particular products through the melt cooler not only may be unnecessary, but could also be undesirable or even problematic. So with this in mind, it has become desirable to have the flexibility to bypass the melt cooler when running certain grades of polymeric materials, and use the melt cooler for other types of materials.
One possible method of accomplishing the aforementioned bypass mode of operation is to remove the melt cooler from the process line. Removal of the melt cooler, however, requires both substantial labor and time to change out and/or to re-install. Removal of the melt cooler also requires special adapter plates for connecting the piping, along with short versions (i.e., for normal mode of operation) and long versions (i.e., for bypass mode of operation) of interconnecting wires and pipes. Removal of the melt cooler can also require special track or rail systems on the floor to guide the equipment out of and back into place. Optionally, a “spool” can be inserted in place of the melt cooler, i.e., to connect the piping upstream of the cooler with the piping downstream of the cooler. A spool is a straight large bore pipe with or without any coolant connection, so that adapters, wiring, or piping need not be changed so often.
Another prior art method of cooling is shown in FIG. 3. A diverter valve 20 is included in the process line upstream of the melt cooler 22 and routes the melt into a bypass line 24 running parallel to the melt cooler 22. Another valve 26 is installed downstream of the melt cooler 22 in order to return the product to the process line. One disadvantage of this option is that it requires a longer overall process line. Two additional high pressure valves 20 and 26 are also required, and a long hollow tube pipe is needed for the bypass line 24. The bypass line 24 also must be rated for high pressure and must be heated to maintain the temperature of the melt. The interior of bypass line 24 may also require static mixers, and line 24 will contain product inventory, which is a consideration for cleaning and changeover of the mode of operation.